This invention relates to high solids coating compositions. In the area of automotive coatings, a coating is typically desired which provides not only a lustrous or glamourous appearance but also good corrosion protection. In order to achieve the desired effect, the automobile is typically provided with several coating layers. Therefore, usually a pigmented coating composition is first applied followed by a transparent topcoat. By transparent is meant a clear coating, that is one which does not contain pigmentation or contains only transparent pigments. Such a coating system is commonly referred to as a "clear-over-color" or "color plus clear".
A very important feature required of color plus clear systems is good "strike-in" ("soak-in") resistance of the basecoat. By "strike-in" resistance is meant the ability of the basecoat compposition of a multicoat system to resist attack by the solvents in the topcoat composition. The topcoat "striking-in" to the basecoat is particularly undesirable since it adversely affects alignment of the metallic pigment generally found in the basecoat and reduces or eliminates the "flop" and the gloss of the topcoat. By "flop" is meant the visual change in brightness or lightness of the metallic aluminum flake with a change in the viewing angle. That is, a change of from 90 to 180 degrees. The greater the visual change from light to dark appearance, the better the flop. The flop accentuates the lines and curves of an automobile; therefore, it is very important in achieving the desired appearance of the coating.
The "strike-in" is a problem because the automobile manufacturers generally wish to apply the topcoat composition to the basecoat composition in a color-plus-clear system by a "wet-on-wet" technique. By this is meant that the basecoat composition is applied to the substrate followed by the topcoat composition and then a single baking step is utilized to cure the composite coating. This problem of topcoat "striking in" to the basecoat is especially pronounced with high solids clear topcoating compositions which are formulated from low molecular weight polymers.
Many different approaches have been utilized to remedy the problem of the topcoat "striking-in" to the basecoat with various degrees of success. One approach has been the use of different rheology additives. Some of these have been disadvantageous in several respects including diminished sprayability at the desired high solids level as well as detrimental effects to the flop because of darkening of the topcoating composition.
There is a need therefore for a way to improve the "strike-in" resistance of basecoating compositions, particularly when applied wet-on-wet with high solids clear topcoating compositions, without detrimental effects to flop and sprayability at the desired high solids levels.